The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to an improved method of electrically connecting a cathode to a cathode lead in a capacitor and an improved capacitor formed thereby.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7′,8,8′-tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers all dielectric surfaces. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. In addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such as silver filled adhesive; and a highly conductive metal lead frame. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The carbon layer serves as a chemical barrier between the solid electrolyte and the silver layer. Critical properties of the layer include adhesion to the underlying layer, wetting of the underlying layer, uniform coverage, penetration into the underlying layer, bulk conductivity, interfacial resistance, compatibility with the silver layer, buildup, and mechanical properties.
The silver layer serves to conduct current from the lead frame to the cathode and around the cathode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and acceptable mechanical properties. Compatibility with the subsequent layers employed in the assembly and encapsulation of the capacitor are also critical. In the case where a silver filled adhesive is used to attach to a lead frame compatibility between the lead frame and the silver filled adhesive is necessary. In capacitors which utilize solder to connect to the external lead, solderability and thermal stability are important factors. In order for the solder to wet the silver layer, the resin in the silver must degrade below the temperature at which the solder is applied. However, excessive degradation of the resin creates an effect termed “silver leeching” resulting in a poor connection between the external cathode layers and the external cathode lead. The traditional approach to applying a silver layer requires a delicate compromise in thermal stability of the resin in order to simultaneously achieve solder wetting and to avoid silver leeching. The silver layer is secured to a cathode lead frame by an adhesive. The adhesive is typically a silver filled resin which is cured after the capacitor is assembled.
Reliability of the capacitors requires that the interface between the silver layer and carbon layer and the interface between the silver layer and adhesive layer have good mechanical integrity during thermo mechanical stresses. Solid electrolytic capacitors are subject to various thermomechanical stresses during assembly, molding, board mount reflow etc. A weak interface with the silver layer can cause delamination of the layers which causes reliability issue. Solid electrolytic capacitors are also required to have good environmental properties such as good chemical and moisture resistance. Reliability issues caused by silver migration under humid conditions are known in the electronics industry. Silver metal from the silver layer can migrate to the anode causing high leakage current. Due to these problems, elimination of the silver layer is desired by solid electrolytic capacitor manufacturers. Other methods for forming a conductive metal surface on the carbon layer are sputtering and electroplating.
U.S. Pat. Nos. 4,000,046, and 4,104,704 teach an electroplating method for solid electrolytic capacitors. Electroplating was performed on water based graphite coatings and silver paint coatings. Experiments to reproduce the method of this disclosure showed significant reliability issues such as high leakage current and electrical shorts. Investigations to understand this suggest that the diffusion of the plating electrolyte through this hydrophilic and porous conductive layer to the semi conductive layer and anode is influencing the reliability. It is also found that the top of the anode with no carbon layer provides significantly more permeability for the plating electrolyte diffusion. Through diligent research the present inventor has developed an electroplating method which circumvents the problems encountered in the prior art.